The present invention relates to an electric arc welder of the type using two consumable electrodes for welding the joint between two edges of relatively thick plates, which plates may be in the form of the cylindrical ends of two pipe sections being welded together in a pipe welding operation or, in practice, seam welding of a formed plate into a pipe. In such welding procedures, two tandem mounted consumable electrodes are moved in unison along the joint so two layers of molten metal are deposited first from the front electrode and then from the trailing electrode to fill the joint between the two beveled edges of the adjacent edges of the plate forming a pipe. These edges are hereinafter referred to as adjacent plates for global application. The invention is applicable for seam welding of pipe; however, pipe welding will be described in a welding operation where the ends of the pipe sections are welded together. The invention is much broader in application and may be used to butt weld two adjacent heavy plates, such as the plates forming sections of gantries for oil rigs, armor of plates used in ship building or seam welding of pipe. Tandem consumable electrodes deposit large amounts of molten metal and cause the metal to be fused in the joint between the plates at high rates as necessary in welding heavy plates. In the past, a single phase power supply was normally used to produce the welding current for both of the tandem electrodes. The frequencies of the welding operation for the adjacent electrodes were the same causing extreme arc generated interference. Such systems required elaborate connections, for instance a Scott connection that produce an electrical phase shift. The frequencies of the welding procedures were dictated by the line frequency of the single phase input power supply. Thus, the frequencies of the pulsating welding current for the tandem mounted electrodes was generally the same fixed value determined by the 50 Hz or 60 Hz input voltage. The prior art welders using tandem consumable electrodes had generally caused an unbalance in the three phase power system and had welding frequencies controlled by the line frequency of the input voltage. This limitation was extremely detrimental when the high currents of pipe welding were used, which high currents exceed about 200 amperes and were often as high as about 1000-1200 amperes, or more. When tandem mounted electrodes are subject to relatively fixed low frequencies, determined by the line frequency, and conduct extremely high currents, used in pipe welding and welding thick plates, arc interference presents a serious problem requiring complicated connections and shielding. In the prior devices, it was somewhat normal practice for each of the electrodes to be driven by current having the same frequency, such as 50 Hz or 60 Hz. The only way to reduce arc interference was to shift the phase of the line current being directed to each of the tandem electrodes. This procedure was extremely complicated. The process was dictated by the line frequency and the phase shifting did not fully alleviate arc interference. Increasing the frequency of the welding current above 100-200 Hz to reduce interference was not practical in the prior systems. Thus, there is a substantial need for an improved electric arc welder using two or more consumable electrodes which do not have the problem of arc interference and do not utilize high current pulse frequencies.